It is well known that diisocyanates which are liquid at room temperature (i.e., about 25.degree. C) have numerous advantages over solid diisocyanates because they are easier to mix and work with. However, diisocyanates which are liquid at room temperature and which are used on a large technical scale, such as toluene diisocyanate or hexamethylene diisocyanate, are as a rule physiologically harmful due to their high vapor pressure and therefore can only be used if certain safety precautions are taken. For this reason, various attempts have been made, either to start with diisocyanates that are normally liquid at room temperature and to reduce their physiological effects by certain procedures or to start with diisocyanates that are solid at room temperature and to convert these into liquid form. In both cases, however, one usually obtains either isocyanates of higher valency, i.e., tri- or polyisocyanates or higher molecular weight diisocyanates or a combination of these effects.
The most important diisocyanates which are solid at room temperature and which are readily available on a large commercial scale are 4,4'-diphenylmethane diisocyanate and the 2,4'-isomer thereof which melt at 39.degree. C and 34.5.degree. C respectively. Attempts have already been made to liquify both the 4,4'-diphenylmethane diisocyanate and a mixture of the 4,4'-diphenylmethane diisocyanate and a small amount of the 2,4'-isomer. Thus, for example, in U.S. Pat. No. 3,644,457, 1 mol of a diphenylmethane diisocyanate is reacted with from about 0.1 to about 0.3 mols of poly-1,2-propylene ether glycol. While the products made according to this patent have met with commercial success, they still suffer from a serious drawback. Specifically, it has been found that these adducts generally will crystallize anywhere from 5.degree. C to as high as 25.degree. C. In fact, when supplied in commercial quantities, these adducts are generally transported in heated trucks. Additionally, in order to thaw the materials it is generally necessary to heat them to somewhere in excess of 50.degree. to 60.degree. C. While in warmer climates, there may not be any problem, in colder areas where the product may be stored in tanks over a period of time, this tendency to crystallize can become a very serious problem. Similar attempts to form liquid diphenylmethane diisocyanates have been described for example, in U.S. Pat. Nos. 3,384,653 and 3,394,164. The attempts to liquify in both of these instances were based on the addition of, in one case, a trihydrocarbyl phosphate, and, in the other case, small amounts of phosphoric acid. In any event, the storage stability of both of these types of products in again quite good around room temperature, but as the temperature decreases, both types of materials tend to crystallize.
In U.S. application Ser. No. 766,997, filed on Feb. 9, 1977, diisocyanates are described which are both stable and liquid at -5.degree. C for at least 48 hours. The diisocyanates disclosed therein are produced by reacting a diphenylmethane containing at least 15 percent by weight of the 2,4'-isomer with a propylene glycol or poly-1,2-propylene ether glycol. Although such products represent an important advance in the art, the search has continued for other liquid organic diisocyanates.
It is therefore an object of this invention to provide improved liquid organic diisocyanates which are liquid and stable at temperatures lower than room temperature. A further object of this invention is to provide organic diisocyanates which remain liquid even on prolonged storage at low temperatures.